Method for determining the quality of a communication channel

ABSTRACT

In accordance with the present invention, there are provided methods for estimating the level of interferers and the level of a useful signal in a spectral window of a given bandwidth in a communication channel in a complex multipath propagation environment. The first method of estimating the level of interferers comprises generating and transmitting several sets of sine waves on the Nyquist frequency. Each set is characterized by a certain phase so that the sequence of phase values uniformly sweeps the interval (0, 360).The second method comprises generating and transmitting several sets of sine waves of the same phase, with frequencies uniformly sweeping a vicinity of the Nyquist frequency. The method of estimating the level of a useful signal comprises transmitting several repetitions of the signal used for actual data transmission and processing it by the receiving station.

FIELD OF THE INVENTION

The present invention relates to determining the quality of a communication channel and, more particularly, to reliably estimating signal to noise ratio of the channel in a complex multipath propagation environment.

BACKGROUND OF THE INVENTION

It is recognized in digital communications that a precise knowledge of the parameters characterizing the quality of a communication channel is extremely important for efficient data transmission. This is related to the classical Shannon result stating that the data rate of a communication channel is a function of the signal to noise ratio (SNR). For a given signal, the increase in the noise level affects the quality of transmission adversely. The amount of information that can be reliably transmitted over a communication channel decreases as the noise increases.

The precise knowledge of the SNR of a channel dictates how much information will be ultimately transmitted. If one underestimates the SNR, the channel efficiency drops compared to the most efficient transmission mode. If one overestimates the SNR then more data is transmitted but the information is unreliable. Therefore, it is extremely important to be able to estimate the SNR of a communication channel in a precise and timely manner.

This is even more important in modern wireless communications that meet new challenges with respect to increasingly complex propagation environments. Typical examples of such environments would be urban areas, office buildings, military vessels, war zones, jungles, rock concerts etc. Some of these environments are characterized by severe multipath effects, others by strong fading, and others by both. When a communication session between two stations is initiated in such an environment, the SNR should be measured on both ends of the channel as precisely as possible and further communications over this channel are stipulated by the SNR value measured.

The present application is related to U.S. Patent Disclosure No. ______, titled “Method and Apparatus for Determining the Quality of a Communication Channel in Complex Propagation Environments”, invented by Vlad Mitlin, filed on Feb. 18, 2004, and owned by the same assignee now and at the time of the invention.

SUMMARY OF THE INVENTION

In accordance with the present invention, there are provided methods for estimating the level of interferers and the level of a useful signal in a spectral window of a given bandwidth in a communication channel in a complex multipath propagation environment. The first method of estimating the level of interferers comprises generating and transmitting several sets of sine waves on the Nyquist frequency. Each set is characterized by a certain phase so that the sequence of phase values uniformly sweeps the interval (0, 360). The second method of estimating the level of interferers comprises generating and transmitting several sets of sine waves of the same phase, with frequencies uniformly sweeping a vicinity of the Nyquist frequency. There is an agreement established between the transmitting and receiving stations determining the duration of time intervals between successive sine wave sets. This duration should not be smaller than the maximum delay in the channel determined from a separate test. The method of estimating the level of a useful signal comprises transmitting several repetitions of the signal used for actual data transmission and processing it by the receiving station.

BRIEF DESCRIPTION OF THE DRAWINGS

A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent, detailed description, in which:

FIG. 1 is an illustrative system for estimating the level of interferers;

FIG. 2 is a set of actual and estimated noise variances obtained at frequencies of 60, 61, 62, 63, 64, 65, 66, 67 Hz by using the system shown in FIG. 1;

FIG. 3 is a set of actual and estimated noise variances obtained at frequencies of 60, 61, 62, 63, 64, 65, 66, 67 Hz by using the system shown in FIG. 1;

FIG. 4 is a set of actual and estimated noise variances obtained at frequencies of 60, 61, 62, 63, 64, 65, 66, 67 Hz by using the system shown in FIG. 1;

FIG. 5 is a baseband system for estimating the level of interferers;

FIG. 6 is an example of a phase sweeping Nyquist (PSN) set;

FIG. 7 is an estimated versus the actual level of interferers obtained by using PSN sets in the system shown in FIG. 5;

FIG. 8 is an estimated versus the actual level of interferers obtained by using PSN sets in the system shown in FIG. 5;

FIG. 9 is a baseband system for estimating the level of interferers;

FIG. 10 is an estimated versus the actual level of interferers obtained by using FSN sets in the system shown in FIG. 5;

FIG. 11 is an estimated versus the actual level of interferers obtained by using FSN sets in the system shown in FIG. 5;

FIG. 12 is an estimated versus the actual level of interferers obtained by using FSN sets in the system shown in FIG. 5;

FIG. 13 is a complete system for estimating the level of interferers;

FIG. 14 is a noise spectrum in the spectral window scanned;

FIG. 15 is an output of the averager in the system shown in FIG. 13;

FIG. 16 is an estimated versus the actual level of interferers obtained by using PSN sets in the system shown in FIG. 13;

FIG. 17 is an estimated versus the actual level of interferers obtained by using PSN sets in the system shown in FIG. 13;

FIG. 18 is an estimated versus the actual level of interferers obtained by using PSN sets in the system shown in FIG. 13;

FIG. 19 is an estimated versus the actual level of interferers obtained by using FSN sets in the system shown in FIG. 13;

FIG. 20 is an estimated versus the actual level of interferers obtained by using FSN sets in the system shown in FIG. 13;

FIG. 21 is an estimated versus the actual level of interferers obtained by using FSN sets in the system shown in FIG. 13;

FIG. 22 is an estimated versus the actual level of interferers obtained by using FSN sets in the system shown in FIG. 13;

FIG. 23 is an example of a frequency sweeping Nyquist (FSN) set;

FIG. 24 is a baseband system for estimating the level of a useful signal;

FIG. 25 is an estimated and the actual level of interferers versus the number of repetitions obtained by using the system shown in FIG. 24; and

FIG. 26 is a flowchart of the method described.

For purposes of clarity and brevity, like elements and components will bear the same designations and numbering throughout the FIGURES.

DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention introduces a new method of determining the level of interferers in a spectral window of a given bandwidth in a communication channel established between the transmitting and receiving stations. The main idea of the method is to generate a special baseband signal at the transmitting station. This signal may be one of two kinds. The first is a sequence of sine wave sets with frequencies equal to the Nyquist frequency of the spectral window and phases uniformly sweeping the interval of (0, 360). The second is a sequence of sine wave sets of the same phase and frequencies uniformly sweeping a vicinity of the Nyquist frequency of the spectral window. Below, this idea is illustrated by a set of simulations of increasing complexity.

1. Main Idea Illustrated

FIG. 1 shows the simplest arrangement. The propagation environment is described by a multipath channel model with 10 paths, the maximum delay value of 0.3 seconds, and the value of the K-factor of 1. This severe environment is aggravated by adding Gaussian white noise (AWGN). Depending on the intricacy of the transmitter and receiver the multipath distortion of a signal may or may not be a negative effect. Specifically, if the RAKE receiver is used, the information contained in indirect paths will be useful and actually will improve the quality of the signal received. If the receiver doesn't have such capabilities the multipath distortion presents a negative effect. Of course, the presence of AWGN is always a negative factor for transmission.

Consider a baseband channel sampled with the frequency of 128 Hz. The test for determining the noise level comprises transmitting several sets of sine waves of increasing frequency and the phase of 0. Seven sets are transmitted while the sine wave frequency changes by one Hz from 60 Hz to 67 Hz. Notice that our simulation describes scanning the spectrum in the vicinity of the Nyquist frequency, which in this particular case, is equal to 64 Hz. FIGS. 2 to 4 present the results. FIG. 2 corresponds to the actual AWGN variance of 1e−14 sq. V; FIG. 3—to 1e−4 sq. V; and FIG. 4—to 1 sq. V. All three figures show both an actual and estimated noise variance versus the frequency of the sine wave. The value of the AWGN variance at the Nyquist frequency measured at the receiver is exactly equal to the actual value. It is especially stunning how different are the variance values estimated at Nyquist and non-Nyquist frequencies. For example, in FIG. 2 the difference is about 14 orders.

2. Phase Sweeping Nyquist Set Test

The simulation described is somewhat idealistic. It assumes that one can perfectly synchronize the transmitter and the receiver, so that there would be no phase shift between the transmitted and the received sine wave. In reality, such a phase shift is very hard to control. Therefore, the following generalization of the method is proposed. The test comprises transmitting several sets of sine waves on the Nyquist frequency. Each set is characterized by a certain phase value of the sine wave. The sequence of phase values uniformly sweeps the interval (0, 360). There is an agreement established between the transmitting station and the receiving station determining the duration of time intervals between successive sine wave sets. This duration should not be smaller than the maximum delay in the channel, determined from a separate test preceding the SNR measurement. In each data set one measures an average square of the magnitude of the received signal. The minimum value of this parameter over all data sets yields an estimate of the level of interferes. Determining the level of interferers, as described above, can be attained by means of transmitting a single wave form termed the Phase Sweeping Nyquist (PSN) set.

FIG. 5 shows the next system considered. This is a continuous system for SNR estimation. Token 0 represents a generator of PSN sets described by the following equation: s(t)=A*sin(pi*f*t+(2*pi*n/N)*int(f*t/n)).  (1)

In this equation s(t) is the signal transmitted; f is the sampling rate (or, equivalently, the two-sided bandwidth of the spectral window where the SNR is to be determined); N is the total number of samples in the test; n is a parameter for determining the number of samples used in the averager (token 7); A is the signal amplitude chosen from the condition of equality of values of an average transmitted power of a PSN set (i.e. half the A value squared) and of a signal used in actual data transmission; pi=3.1415926; and int(x) denotes the maximum integer not exceeding a given real number x. The signal on the receiver is downconverted, sampled, and filtered. Then it is squared and averaged (token 7) with a window of n*D−r seconds, where D=1/f is a spacing between samples; and r is the maximum delay in the channel determined in a separate test. The result goes into the sink (token 9) where the minimum of the average variance is determined. This yields an estimate of the level of interferers.

Parameters for this simulator are as follows: sampling rate 128e+6 Hz, n=256, multipath channel with 1 path, maximum delay of 200e−9 seconds, K-factor of 1, total number of samples in a PSN set N=64n, the amplitude of the PSN set was 1 V; the sampling jitter constant equals D/10. The lowpass filter used is the Butterworth filter with 10 poles and a cutoff frequency of 62e+6 Hz.

FIG. 6 presents the signal given by Eq. (1) transmitted during one SNR test. The PSN set is nearly (but not exactly) periodical with the period of about 1/9 of the test duration.

FIG. 7 presents the results of the SNR test for several different levels of AWGN. The estimated values are in agreement with the actual ones. Note that more precision is lost as the level of noise decreases.

FIG. 8 presents the results of the SNR test where two types of interferers affect the channel, e.g. AWGN and PN-sequence with the rate of 12.8e+6 Hz. The magnitude of the PN-sequence is equal to the m.s.d. of the AWGN. The estimated and the actual data are in excellent agreement. We did not observe any performance deterioration of the SNR estimator at lower noise levels. This is probably due to mutual compensation of error by noises from two sources.

We are able to achieve an even better performance of an SNR estimator, especially in the range of high SNR. The corresponding system is shown in FIG. 9. The wave form transmitted is a PSN set (token 0). The precision of the estimation is improved due to the use of a more complex estimator involving current and delayed samples, instead of just squaring the current signal value. This operation is implemented by token 7 in FIG. 9. The input to this token is the current signal value and the ones delayed by one and two samples, respectively (tokens 6 and 8). Token 7 performs the following calculation: Output=s(t)*(s(t)+2*s(t−D)+s(t−2*D)).  (2)

The estimator using Eqs. (1) and (2) has the precision of about 1 dB in the entire range of noise measurements taken.

At high SNR the time dependence of the noise variance usually has a sharp global minimum. At low SNR this dependence has many local minima with the values close to the value in the global minimum. Respectively, at low SNR this dependence should go through an additional data processing routine to account for these local minima. Firstly, this dependence is raised to a power m: we found that m=32 works well. The next step is a time integration of the result of this transformation. The final step is to divide the result of integration by the total test time and raising the result to a power of 1/m. This data treatment improves results at low SNR and leaves them practically unchanged at high SNR.

3. Frequency Sweeping Nyquist Set Test

This section introduces another method of determining the level of interferers with a single wave form termed Frequency Sweeping Nyquist (FSN) set. The basic idea is not to generate sine waves on a Nyquist frequency while sweeping the entire phase range uniformly but rather sweeping the frequency range in a small vicinity of the Nyquist frequency. Depending on the characteristics of the transmitter, using one of either FSN or PSN set may be preferred.

The system considered below is equivalent to the one shown in FIG. 5 except that token 0 now generates the following wave form: s(t)=sin(pi*f*t*(1−b+(2*n*b/N)*int(f*t/n))).  (3)

In Eq. (3) s(t) is the signal transmitted; f is the sampling frequency; N is the total number of samples in an FSN set; b is the number defining the size of the vicinity of the Nyquist frequency to be swept in the test, (f*(1−b)/2, f*(1+b)/2); n defines the window in the averager in FIG. 5 as nD−r; and A is the FSN set amplitude determined from the condition of equality of values of an average transmitted power of a FSN set and of a signal used in actual data transmission.

FIGS. 10 to 12 present simulation results of an FSN set test. Simulation parameters are: sampling rate 128e6 Hz; N=16,384; n=256; sampler aperture and aperture jitter are both equal to D/4; multipath channel with one path; r=200e−9 seconds; K-factor=1; b=0.01; A=1 V. To obtain each data point the test was run three times as shown in the figures. FIG. 10 presents results in the case of the AWGN only; FIG. 11 corresponds to a channel distortion by two interferers, AWGN and a harmonic wave with the frequency of 12.8e+6 Hz.; FIG. 12 shows the results for a channel distortion caused by AWGN and a chirp with a start frequency 0 Hz, stop frequency 25e6 Hz and the period of 1e−6 seconds. In the last two cases the amplitude of each periodical interferer was equal to the m.s.d. of AWGN. The results show a good agreement between actual and estimated levels of interferers (e.g. the maximum deviation of an estimate from the actual value was never larger than 1 dB).

4. Determining the Noise Level in a Non-Baseband System

This section describes the most convincing proof of the concept presented in this invention. Specifically, we describe the results of simulating the SNR test including up/down-conversion of the baseband signal. FIG. 13 presents the system considered. Token 0 implements the signal generation. In the case of a PSN set it generates the following signal: s(t)=A*sin(2*pi*h*t+pi*f*t+(2*pi*n/N)*int(f*t/n)).  (4)

In Eq. (4) we use the same notations as in Eq. (2); with one new parameter: h, representing the carrier frequency. In the case of an FSN set the signal generated is: s(t)=A*sin(2*pi*h*t+pi*f*t*(1−b+(2*b*n/N)*int(f*t/n)))  (5)

The signal is then transmitted over the multipath channel (token 1), and it is also corrupted by an additive noise (token 8). On the receiver the signal is down-converted by multiplying it by: s(t)=sin(2*pi*h*t+F)  (6)

(token 7); F is a phase shift between signals at the transmitting and receiving stations. Then the signal is sampled (token 6), filtered (token 3), squared (token 4), and averaged (token 5) with a window whose definition was given in earlier sections. The results are collected over the test duration, the minimum is determined which yields an estimate of the noise level (token 12).

The parameters used in simulations are as follows: the carrier is a harmonic wave with an amplitude of 1 V and frequency of 256e+6 Hz. The carrier is sampled at a rate of 1,024e+6 Hz. The baseband signal is sampled at a rate of 128e+6 Hz. Then the Butterworth lowpass filter with 10 poles and a cutoff frequency of 62e+6 Hz is applied. This means that the spectral window scanned is 194e+6 Hz to 318e+6 Hz. The actual level of noise in this window is determined by passing the noise through a bandpass filter token 10). In this set of simulations the aperture and jitter of the sampler presented by token 6 on the receiver are 0.

FIG. 14 shows the Fourier spectrum of the noise at the outlet of token 10. FIG. 15 presents typical data at the outlet of the averager (in logarithmic scale). FIGS. 16 to 22 present the results of the SNR test with AWGN as the only interferer. Note that the variance of the transmitted signal is 0.5 sq. V.

FIG. 16 presents the results of the SNR test using a PSN set; the total number of samples is 16,384 (i.e. the total number of “baseband samples” is 2048). One can see that in the low and intermediate SNR range the typical variation of results is negligible while in high SNR range the variation is considerable. FIG. 17 shows the results with the same parameters except for the total number of samples being 65,536 (or “baseband samples” 8192). Increasing the number of samples makes the variation in the results in high SNR range also negligible. FIG. 18 presents the results with the same set of parameters that was used in generating FIG. 17 except for the sampler parameters: namely, at the receiving station after down-conversion the PSN set is sampled with the the aperture of 1.95e−9 seconds and the jitter constant of 0.97e−9 seconds. The results show a consistent underestimation of the noise variance by about 3 dB in the entire SNR range studied.

FIG. 19 presents the results of an SNR test using an FSN set with b=0.01, the total number of samples was 16,384; and the signal was assumed to be sampled at the receiving station without jitter. FIG. 20 and FIG. 21 show the results with the same parameters except that the total number of samples was 65,536 and 262,144, respectively. Again, an increase in the total number of samples in the wave form used for this test improves the precision of the estimated level of noise in the high SNR range. FIG. 22 presents the results with the same set of parameters that was used in generating FIG. 21 except for the sampler parameters: namely, at the receiving station after down-conversion the FSN set is sampled with the the aperture of 1.95e−9 seconds and the jitter constant of 0.97e−9 seconds. The results show a consistent underestimation of the noise variance by about 3 dB in the entire SNR range studied.

FIG. 23 presents an example of an FSN set generated at the transmitting station.

Also, we have run simulations for the same set of parameters but varying the phase shift F. This was important in order to determine how large the variation in the results would be if the signal detection were performed non-coherently and F would not be controlled. In all cases the effect of choosing F at random did not exceed 0.5 dB.

5. Method of Measuring the Level of Useful Signal

So far, we concentrated on determining the level of interferers in the channel. One also has to determine the level of a useful signal at the receiving station. This is accomplished by transmitting several repetitions of the signal.

FIG. 24 shows a baseband system for this test. FIG. 25 shows how many repetitions of a signal one has to transmit to be able to reliably determine its level. Initial data: sampling rate of 128e6 Hz; multipath channel with one path; maximum delay of 200e−9 seconds; and K-factor of 1; AWGN with m.s.d. of 0.71 V; sampling jitter of D/10. The signal used was a frequency sweep with a amplitude of 1 V; start frequency of 0 Hz; stop frequency of 25e6 Hz; and period of 1e−6 seconds. FIG. 25 shows the dependencies of the difference in measuring variances in tokens 9 and 7 (which should yield an actual noise variance) and the actual noise variance versus the number of signal repetitions. These two dependencies converge as the number of repetitions is about 32.

6. Discussion

This invention introduces a new method for estimating the level of interferers and the level of a useful signal in a communication channel in a complex multipath propagation environment. The method uses a special wave form that can be either the PSN set (Eq. (1)) or the FSN set (Eq. (3)). The method comprises:

-   -   Determining the maximum delay in the channel by sending a pulse         from the transmitting station to the receiving station;     -   Generating either a PSN set (Eq. (1)) or an FSN set (Eq. (3)) at         the transmitting station;     -   Up-converting this signal and transmitting it from the         transmitting station;     -   At the receiving station, down-converting the signal, sampling         it, and passing it through a filter;     -   At the receiving station, squaring the signal; or, in the case         of a PSN set, applying the transformation given by Eq. (2);     -   At the receiving station, averaging the result with a window         smaller than the parameter n in Eqs. (1) or (3) by the maximum         delay in the channel determined earlier;     -   At the receiving station, determining the minimum of the result         over the duration of the test; which determines the level of         noise in the channel;     -   At the transmitting station, sending the signal used for actual         data transmission, several times;     -   At the receiving station, down-converting, sampling, and         filtering the signal;     -   At the receiving station, squaring and averaging the result         which determines the level of a useful signal plus the level of         noise;     -   At the receiving station, determining the level of a useful         signal and, by applying the level of noise value, calculating         the channel SNR.

FIG. 26 presents a flowchart of the method presented.

In this document we present a method of determining the channel quality between two stations. Our method also describes a procedure for determining the quality of data transmission in a network if these two stations are nodes of the network.

7. References

John G. Proakis: Digital Communications, McGraw Hill, N.Y., 1995;

John G Proakis, Masoud Salehi: Contemporary Communication Systems Using Matlab, Brooks/Cole, Pacific Grove, 2000;

Dimitri Bertsekas, Robert Gallager: Data Networks, Prentice Hall, N.J., 1992;

“Energy estimator and method therefore”, U.S. Pat. No. 5,666,429;

“Low complexity model based channel estimation algorithm for fading channels”, U.S. Pat. No. 5,581,580;

“Average signal to noise ratio estimator”, U.S. Pat. No. 6,442,495;

“Method and apparatus for operating an adaptive decision feedback equalizer”, U.S. Pat. No. 6,298,242;

“System and method for bit loading with optimal margin assignment”, U.S. Pat. No. 6,205,410;

Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the example chosen for purposes of disclosure, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention.

Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims. 

1. A method for determining the quality of a communication channel for estimating the channel signal to noise ratio in a complex propagation environment comprising: means for estimating the level of interferers in a communication channel established between the transmitting station and the receiving station; means for estimating the level of interferers in a communication channel established between two nodes in a communication network; means for estimating the level of a useful signal at the receiving station; means for estimating the level of a useful signal at a node of a communication network; means for estimating the signal to noise ratio at the receiving station; means for estimating the signal to noise ratio at a node of a communication network; means for generating phase sweeping nyquist sets for estimating the level of interferers in a communication channel established between the transmitting station and the receiving station; means for generating phase sweeping nyquist sets for estimating the level of interferers in a communication channel established between two nodes of a communication network; means for generating frequency sweeping nyquist sets for estimating the level of interferers in a communication channel established between the transmitting station and the receiving station; means for generating frequency sweeping nyquist sets for estimating the level of interferers in a communication channel established between two nodes in a communication network; means for estimating the level of interferers in a communication channel established between the transmitting station and the receiving station by using phase sweeping nyquist sets; means for estimating the level of interferers in a communication network by using phase sweeping nyquist sets; means for estimating the level of interferers in a communication channel established between the transmitting station and the receiving station by using frequency sweeping nyquist sets; means for estimating the level of interferers in a communication network by using frequency sweeping nyquist sets; means for estimating the level of interferers in a spectral window of a given bandwidth by generating and transmitting several sets of sine waves on the nyquist frequency; means for estimating the level of interferers in a communication network in a spectral window of a given bandwidth by generating and transmitting several sets of sine waves on the nyquist frequency; means for estimating the level of interferers in a spectral window of a given bandwidth by generating and transmitting several sets of sine waves with frequencies uniformly sweeping a vicinity of the nyquist frequency; and means for estimating the level of interferers in a communication network in a spectral window of a given bandwidth by generating and transmitting several sets of sine waves with frequencies uniformly sweeping a vicinity of the nyquist frequency.
 2. The method for determining the quality of a communication channel in accordance with claim 1, wherein said means for estimating the level of interferers in a communication channel established between the transmitting station and the receiving station comprises a means for determining the maximum delay in the channel by sending a pulse from the transmitting station to the receiving station, means for generating, upconverting, and transmitting a signal for determining the level of interferers, means for generating, upconverting, and transmitting a signal for determining the level of interferers by using a special processor, means for generating, upconverting, and transmitting a signal for determining the level of interferers by using lookup tables, means for down-converting said signal, sampling it, and passing it through a filter at the receiving station, means for squaring said signal at the receiving station, means for averaging the result of said squaring at the receiving station, means for determining the minimum of the result of said averaging over the duration of the test; which determines the level of interferers in the channel at the receiving station (claim 2).
 3. The method for determining the quality of a communication channel in accordance with claim 1, wherein said means for estimating the level of interferers in a communication channel established between two nodes in a communication network comprises a method of claim 2, wherein the transmitting station and the receiving station are nodes of a network (claim 3).
 4. The method for determining the quality of a communication channel in accordance with claim 1, wherein said means for estimating the level of a useful signal at the receiving station comprises a means for generating, upconverting, and transmitting the signal used for data transmisison, several times, means for receiving, down-converting, sampling, and filtering said signal, means for squaring and averaging said signal which determines the level of a useful signal plus the level of interferers, means for determining the level of a useful signal by applying the level of interferers determined according to claim 2 (claim 4).
 5. The method for determining the quality of a communication channel in accordance with claim 1, wherein said means for estimating the level of a useful signal at a node of a communication network comprises a method of claim 4, wherein the transmitting station and the receiving station are nodes of a network (claim 5).
 6. The method for determining the quality of a communication channel in accordance with claim 1, wherein said means for estimating the signal to noise ratio at the receiving station comprises a method of estimating the signal to noise ratio from the level of interferers and the level of a useful signal (claim 6).
 7. The method for determining the quality of a communication channel in accordance with claim 1, wherein said means for estimating the signal to noise ratio at a node of a communication network comprises a method of claim 6, wherein the transmitting station and the receiving station are nodes of a network (claim 7).
 8. The method for determining the quality of a communication channel in accordance with claim 1, wherein said means for generating phase sweeping nyquist sets for estimating the level of interferers in a communication channel established between the transmitting station and the receiving station comprises a method for generating the signal for determining the level of interferers being the phase sweeping Nyquist set generated by applying the formula: s(t)=sin(pi*f*t+(2*pi*n/N)*int(f*t/n)), where s(t) is the signal transmitted; f is the sampling rate; N is the total number of samples in the test; n is the number of samples used in the averager plus the number of samples in a time window equal to the maximum delay in the channel; A is the amplitude of the signal; int(x) denotes the maximum integer not exceeding a given real number x; pi=3.1415926 (claim 8).
 9. The method for determining the quality of a communication channel in accordance with claim 1, wherein said means for generating phase sweeping nyquist sets for estimating the level of interferers in a communication channel established between two nodes of a communication network comprises a method of claim 8, wherein the transmitting station and the receiving station are nodes of a network (claim 9).
 10. The method for determining the quality of a communication channel in accordance with claim 1, wherein said means for generating frequency sweeping nyquist sets for estimating the level of interferers in a communication channel established between the transmitting station and the receiving station comprises a method for generating the signal for determining the level of interferers being the frequency sweeping Nyquist set generated by applying the formula: s(t)=sin(pi*f*t*(1−b+2*b*n/N)*int(f*t/n)), where s(t) is the signal transmitted; f is the sampling rate; N is the total number of samples in the test; n is the number of samples used in the averager plus the number of samples in a time window equal to the maximum delay in the channel; A is the amplitude of the signal; b is the number defining the size of the vicinity of the Nyquist frequency to be swept in the test, (f*(1−b)/2, f*(1+b)/2); int(x) denotes the maximum integer not exceeding a given real number x; pi=3.1415926 (claim 10).
 11. The method for determining the quality of a communication channel in accordance with claim 1, wherein said means for generating frequency sweeping nyquist sets for estimating the level of interferers in a communication channel established between two nodes in a communication network comprises a method of claim 10, wherein the transmitting station and the receiving station are nodes of a network (claim 11).
 12. The method for determining the quality of a communication channel in accordance with claim 1, wherein said means for estimating the level of interferers in a communication channel established between the transmitting station and the receiving station by using phase sweeping nyquist sets comprises a method of estimating the level of interferers in a communication channel established between the transmitting station and the receiving station by using phase sweeping Nyquist sets, in accordance to claim 8 (claim 12).
 13. The method for determining the quality of a communication channel in accordance with claim 1, wherein said means for estimating the level of interferers in a communication network by using phase sweeping nyquist sets comprises a method of claim 12, wherein the transmitting station and the receiving station are nodes of a network (claim 13).
 14. The method for determining the quality of a communication channel in accordance with claim 1, wherein said means for estimating the level of interferers in a communication channel established between the transmitting station and the receiving station by using frequency sweeping nyquist sets comprises an estimating the level of interferers in a communication channel established between the transmitting station and the receiving station by using frequency sweeping Nyquist sets, in acordance to claim 10 (claim 14).
 15. The method for determining the quality of a communication channel in accordance with claim 1, wherein said means for estimating the level of interferers in a communication network by using frequency sweeping nyquist sets comprises a method of claim 14, wherein the transmitting station and the receiving station are nodes of a network (claim 15).
 16. The method for determining the quality of a communication channel in accordance with claim 1, wherein said means for estimating the level of interferers in a spectral window of a given bandwidth by generating and transmitting several sets of sine waves on the nyquist frequency comprises a method of estimating the level of interferers consisting in generating and transmitting several sets of sine waves on the Nyquist frequency, each set being characterized by a certain phase, the sequence of phase values uniformly sweeping the interval (0, 360), an agreement established between the transmitting and receiving station determining the duration of time intervals between successive sine wave sets; said duration being not smaller than the maximum delay in the channel determined from a separate test (claim 16).
 17. The method for determining the quality of a communication channel in accordance with claim 1, wherein said means for estimating the level of interferers in a communication network in a spectral window of a given bandwidth by generating and transmitting several sets of sine waves on the nyquist frequency comprises a method of claim 16, wherein the transmitting station and the receiving station are nodes of a network (claim 17).
 18. The method for determining the quality of a communication channel in accordance with claim 1, wherein said means for estimating the level of interferers in a spectral window of a given bandwidth by generating and transmitting several sets of sine waves with frequencies uniformly sweeping a vicinity of the nyquist frequency comprises a method of estimating the level of interferers consisting in generating and transmitting several sets of sine waves of the same phase, with frequencies uniformly sweeping a vicinity of the Nyquist frequency, an agreement established between the transmitting and receiving station determining the duration of time intervals between successive sine wave sets; said duration being not smaller than the maximum delay in the channel determined from a separate test (claim 18).
 19. The method for determining the quality of a communication channel in accordance with claim 1, wherein said means for estimating the level of interferers in a communication network in a spectral window of a given bandwidth by generating and transmitting several sets of sine waves with frequencies uniformly sweeping a vicinity of the nyquist frequency comprises a method of claim 18, wherein the transmitting station and the receiving station are nodes of a network (claim 19).
 20. A method for determining the quality of a communication channel for estimating the channel signal to noise ratio in a complex propagation environment comprising: a means for determining the maximum delay in the channel by sending a pulse from the transmitting station to the receiving station, means for generating, upconverting, and transmitting a signal for determining the level of interferers, means for generating, upconverting, and transmitting a signal for determining the level of interferers by using a special processor, means for generating, upconverting, and transmitting a signal for determining the level of interferers by using lookup tables, means for down-converting said signal, sampling it, and passing it through a filter at the receiving station, means for squaring said signal at the receiving station, means for averaging the result of said squaring at the receiving station, means for determining the minimum of the result of said averaging over the duration of the test; which determines the level of interferers in the channel at the receiving station (claim 2), for estimating the level of interferers in a communication channel established between the transmitting station and the receiving station; a method of claim 2, wherein the transmitting station and the receiving station are nodes of a network (claim 3), for estimating the level of interferers in a communication channel established between two nodes in a communication network; a means for generating, upconverting, and transmitting the signal used for data transmisison, several times, means for receiving, down-converting, sampling, and filtering said signal, means for squaring and averaging said signal which determines the level of a useful signal plus the level of interferers, means for determining the level of a useful signal by applying the level of interferers determined according to claim 2 (claim 4), for estimating the level of a useful signal at the receiving station; a method of claim 4, wherein the transmitting station and the receiving station are nodes of a network (claim 5), for estimating the level of a useful signal at a node of a communication network; a method of estimating the signal to noise ratio from the level of interferers and the level of a useful signal (claim 6), for estimating the signal to noise ratio at the receiving station; a method of claim 6, wherein the transmitting station and the receiving station are nodes of a network (claim 7), for estimating the signal to noise ratio at a node of a communication network; a method for generating the signal for determining the level of interferers being the phase sweeping Nyquist set generated by applying the formula: s(t)=sin(pi*f*t+(2*pi*n/N)*int(f*t/n)), where s(t) is the signal transmitted; f is the sampling rate; N is the total number of samples in the test; n is the number of samples used in the averager plus the number of samples in a time window equal to the maximum delay in the channel; A is the amplitude of the signal; int(x) denotes the maximum integer not exceeding a given real number x; pi=3.1415926 (claim 8), for generating phase sweeping nyquist sets for estimating the level of interferers in a communication channel established between the transmitting station and the receiving station; a method of claim 8, wherein the transmitting station and the receiving station are nodes of a network (claim 9), for generating phase sweeping nyquist sets for estimating the level of interferers in a communication channel established between two nodes of a communication network; a method for generating the signal for determining the level of interferers being the frequency sweeping Nyquist set generated by applying the formula: s(t)=sin(pi*f*t*(1−b+2*b*n/N)*int(f*t/n)), where s(t) is the signal transmitted; f is the sampling rate; N is the total number of samples in the test; n is the number of samples used in the averager plus the number of samples in a time window equal to the maximum delay in the channel; A is the amplitude of the signal; b is the number defining the size of the vicinity of the Nyquist frequency to be swept in the test, (f*(1−b)/2, f*(1+b)/2); int(x) denotes the maximum integer not exceeding a given real number x; pi=3.1415926 (claim 10), for generating frequency sweeping nyquist sets for estimating the level of interferers in a communication channel established between the transmitting station and the receiving station; a method of claim 10, wherein the transmitting station and the receiving station are nodes of a network (claim 11), for generating frequency sweeping nyquist sets for estimating the level of interferers in a communication channel established between two nodes in a communication network; a method of estimating the level of interferers in a communication channel established between the transmitting station and the receiving station by using phase sweeping Nyquist sets, in accordance to claim 8 (claim 12), for estimating the level of interferers in a communication channel established between the transmitting station and the receiving station by using phase sweeping nyquist sets; a method of claim 12, wherein the transmitting station and the receiving station are nodes of a network (claim 13), for estimating the level of interferers in a communication network by using phase sweeping nyquist sets; an estimating the level of interferers in a communication channel established between the transmitting station and the receiving station by using frequency sweeping Nyquist sets, in acordance to claim 10 (claim 14), for estimating the level of interferers in a communication channel established between the transmitting station and the receiving station by using frequency sweeping nyquist sets; a method of claim 14, wherein the transmitting station and the receiving station are nodes of a network (claim 15), for estimating the level of interferers in a communication network by using frequency sweeping nyquist sets; a method of estimating the level of interferers consisting in generating and transmitting several sets of sine waves on the Nyquist frequency, each set being characterized by a certain phase, the sequence of phase values uniformly sweeping the interval (0, 360), an agreement established between the transmitting and receiving station determining the duration of time intervals between successive sine wave sets; said duration being not smaller than the maximum delay in the channel determined from a separate test (claim 16), for estimating the level of interferers in a spectral window of a given bandwidth by generating and transmitting several sets of sine waves on the nyquist frequency; a method of claim 16, wherein the transmitting station and the receiving station are nodes of a network (claim 17), for estimating the level of interferers in a communication network in a spectral window of a given bandwidth by generating and transmitting several sets of sine waves on the nyquist frequency; a method of estimating the level of interferers consisting in generating and transmitting several sets of sine waves of the same phase, with frequencies uniformly sweeping a vicinity of the Nyquist frequency, an agreement established between the transmitting and receiving station determining the duration of time intervals between successive sine wave sets; said duration being not smaller than the maximum delay in the channel determined from a separate test (claim 18), for estimating the level of interferers in a spectral window of a given bandwidth by generating and transmitting several sets of sine waves with frequencies uniformly sweeping a vicinity of the nyquist frequency; and a method of claim 18, wherein the transmitting station and the receiving station are nodes of a network (claim 19), for estimating the level of interferers in a communication network in a spectral window of a given bandwidth by generating and transmitting several sets of sine waves with frequencies uniformly sweeping a vicinity of the nyquist frequency. 